Conventionally, serial printing apparatuses which execute printing by using various printing methods in accordance with print data transferred from a host apparatus are widely used. Especially a printing apparatus employing a dot matrix method forms a desired printed image on a sheet-like printing medium by alternately repeating an operation of moving a carriage with a printhead capable of dot printing and an operation of conveying the printing medium in a direction perpendicular to the carriage moving direction. The carriage moving direction is called a main scanning direction, and the direction perpendicular to the main scanning direction is called a sub-scanning direction. A printhead according to this method is widely used in various fields because high-density printing at a relatively high speed can be achieved at a low cost.
In such a serial printing apparatus where a plurality of printing elements for dot printing are arrayed in the sub-scanning direction, it has been known that an unevenness in the printing density occurs because of, e.g., errors between the individual printing elements in the manufacture.
To suppress such density unevenness, in the conventional method, for example, every other elements of the plurality of arrayed printing elements are used for dot printing in the first carriage scanning. The printing medium is conveyed by almost ½ the normal conveyance amount. Next dot printing is done using printing elements that are unused in the first carriage scanning. Printing control is executed by repeating the above-described operations.
FIG. 23 is a perspective view showing the schematic arrangement of a printing apparatus having an inkjet printhead.
As shown in FIG. 23, a carriage 106 that reciprocally moves in the X direction has four ink cartridges 101. The ink cartridges 101 includes ink tanks storing four color inks, i.e., black (K), cyan (C), magenta (M), and yellow (Y) inks and a printhead 102.
FIG. 24 is a view showing ink orifices arrayed on the printhead 102.
FIG. 24 shows the ink orifices of the printhead viewed from the Z direction in FIG. 23. Referring to FIG. 24, reference numeral 201 denotes a plurality of orifices arrayed on the printhead 102. FIG. 24 shows an example with eight ink orifices (n1 to n8) for the descriptive convenience. Four arrays of ink orifices are arranged on the printhead 102 in correspondence with the number of ink colors. FIG. 24 illustrates only one array of orifices.
Referring back to FIG. 23, a conveyance roller 103 rotates in the direction of an arrow in FIG. 23 while holding a printing paper sheet P together with an auxiliary roller 104, thereby conveying the printing paper sheet P in the Y direction as needed. Feed rollers 105 feed the printing paper sheet and also serve to hold the printing paper sheet P, like the conveyance roller 103 and auxiliary roller 104. The carriage 106 stands by in a home position (h) indicated by the dotted line in FIG. 23 when no print operation is executed, or a printhead recovery operation is executed.
Upon receiving a print start instruction, the carriage 106 that is located in the home position before the start of printing executes printing by discharging an ink from the plurality of orifices 201 on the printhead 102 while moving in the X direction. When printing corresponding to print data until the end of the paper sheet is ended, the carriage 106 returns to the home position and executes printing in the X direction again.
To print, e.g., an image, various factors such as color development, tonality, and uniformity have to be considered. Especially for uniformity, a small variation between nozzles in the manufacturing process of the printhead influences the discharge amount or discharge direction of ink from each nozzle in printing. This finally causes a density unevenness of a printed image and degrades the image quality, as is known.
A detailed example will be described with reference to FIGS. 25 and 26.
FIG. 25 is a view showing a state wherein printing is executed by normally discharging ink from a printhead.
FIG. 26 is a view showing a state wherein ink is not normally discharged from a printhead. This means that a printing failure occurs, resulting in density unevenness. Referring to FIGS. 25 and 26, reference numeral 31 denotes a printhead having eight nozzles; 32, nozzles which discharge ink; and 33, ink droplets discharged from the nozzles 32.
Referring to FIG. 25, a indicates that ink droplets are ideally discharged in the same discharge amount in the same direction. If discharge is done in this way, dots with the same size stick to a printing medium such as a printing paper sheet, as indicated by b in FIG. 25. Referring to FIG. 25, c indicates a change in density of print dots in a direction along the nozzle array. When normal ink discharge is done, an overall uniform image without density unevenness can be obtained, as indicated by c in FIG. 25.
Actually, however, the ink discharge characteristic varies between nozzles, as described above. Hence, if printing is done, the sizes or discharge directions of ink droplets discharged from the nozzles vary, as indicated by a in FIG. 26. The discharged ink droplets stick to a printing medium as indicated by b in FIG. 26. According to b in FIG. 26, a blank part that cannot satisfy an area factor of 100% is periodically formed in the main scanning direction, or conversely, the dots overlap more than necessary. In addition, a white stripe is formed at the center of b in FIG. 26. The aggregation of dots sticking to the printing medium in this way exhibits a density distribution indicated by c in FIG. 26 in the nozzle array direction. Such a density distribution is eventually perceived by human eyes as a density unevenness. A stripe generated by a variation in printing medium conveyance amount may also be noticeable.
Japanese Patent Publication Laid-Open No. 6-143618 proposes the following method as a measure to prevent the density unevenness. This method will be described briefly with reference to FIGS. 26 and 27.
FIG. 27 is a view showing a state wherein printing is executed by multi-pass printing.
According to this method, the printhead 31 is scanned three times to complete a printing region shown in FIGS. 26 and 27, as indicated by, e.g., a in FIG. 27. A region of four pixels in the sub-scanning direction, which is ½ the print width of the printhead 31, is completed by two passes. In this case, the eight nozzles of the printhead are divided into two groups: the four upper nozzles and the four lower nozzles. Dots printed by one nozzle in one scanning are obtained by sampling predetermined image data to about ½ in accordance with a predetermined image data sequence. Dots are printed by using the ½ remaining image data in the second scanning, thereby completing the 4-pixel region in the sub-scanning direction.
The above-described printing method is multi-pass printing.
When this printing method is used, even when a printhead with a variation in ink discharge characteristic between the nozzles, as shown in FIG. 26, is used, the influence of the characteristic unique to each nozzles on the printed image can be reduced by half. Hence, a printed image as indicated by b in FIG. 27 is obtained. As a result, the black or white string indicated by b in FIG. 26 is relatively unnoticeable. Hence, the density unevenness is also considerably relaxed as indicated by c in FIG. 27 as compared to c in FIG. 26.
In this multi-pass printing, image data is divided for the first scanning and second scanning in accordance with a predetermined sequence so that the data can complement each other. For this, an image data sequence that forms a staggered pattern every other pixel in the vertical and horizontal directions, as shown in FIGS. 28A to 28C, is generally used.
In the unit printing region (in this example, the 4-pixel region in the sub-scanning direction), printing is completed by the first scanning to print a staggered pattern and the second scanning to print an inverted (complementary) staggered pattern. FIGS. 28A to 28C show how to print a predetermined region by using staggered and inverted staggered sampling patterns (mask patterns).
Referring to FIGS. 28A to 28C, in the first scanning, the staggered sampling pattern is printed by using the four lower nozzles of the printhead 31, as shown in FIG. 28A. In the second scanning, the printing medium is conveyed by a length corresponding to four pixels (½ the print width of the printhead), and the inverted (complementary) staggered sampling pattern is printed, as shown in FIG. 28B. In the third scanning, the printing medium is conveyed by a length corresponding to four pixels (½ the print width of the printhead) again, and the staggered sampling pattern is printed again, as shown in FIG. 28C. In this way, conveyance for four pixels and printing of the staggered and inverted (complementary) staggered sampling patterns are sequentially alternately executed, thereby completing the 4-pixel printing region in the sub-scanning direction by every scanning of the printhead.
Japanese Patent Publication Laid-Open No. 10-175333 also discloses mask patterns applicable to such a multi-pass printing. Especially, Japanese Patent Publication Laid-Open No. 10-175333 describes an arrangement in which a mask pattern is transferred to a printer and registered in it prior to printing. Then, multi-pass printing is performed while applying the mask.
In the above-described prior arts, however, print data is transferred from the host to the printing apparatus on the assumption that printing corresponding to the entire region of one scanning of the printhead is achieved by one scanning of the printhead. On the other hand, the printing apparatus holds the print data so that desired dot printing is executed by a plurality of number of times of scanning, e.g., two scanning operations in the above-described example.
In other words, print data that is not used for printing by one scanning is transferred from the host to the printing apparatus. Especially, a printing apparatus that must execute high-resolution printing requires a large amount of print data for it, and a high speed print data transfer is necessary. Even a printing apparatus on the receiving side must have a high-performance interface to cope with such a high speed transfer and also a large capacity of print buffer to store a large amount of print data. These factors increase the production cost of the apparatuses.
To solve this problem, a technique has been proposed in which print data to be used for each pass printing is transferred from a host to a printing apparatus that executes multi-pass printing. Particularly, to enable a printer and a host to use an interface with a low data transfer rate, Japanese Patent No. 3,229,526 describes a technique of compressing print data to be used for each pass printing and transferring the data to a printing apparatus. According to Japanese Patent No. 3,229,526, all of mask patterns (2×2 staggered patterns) to be applied to multi-pass printing are stored in a memory of the printing apparatus in advance.
However, in a case where a memory capacity integrated with a printing apparatus is small, all of mask patterns necessary for multi-pass printing cannot always be stored in the printing apparatus in advance. More specifically, according to Japanese Patent No. 3,229,526, since a mask pattern (staggered pattern) common to each scan is used, an amount of data for the mask pattern is rather small. For this reason, it is possible to store all of mask patterns (staggered patterns) in a printing apparatus. However, if a printing apparatus changes a mask pattern for each scan, a total amount of mask patterns is huge. Therefore, it is not possible to store all of mask patterns into a small capacity memory. Hence, a multi-pass printing technique capable of allowing even a printing apparatus with a small memory capacity to execute high-resolution high-quality printing at a high speed has been demanded.